Thick-film thermal printhead

ABSTRACT

A thick-film thermal printhead comprises: an oblong rectangular substrate (1) having at least one longitudinal edge (1a); a partial glaze layer provided on the substrate along the longitudinal edge; a linear heating resistor (11) formed on the partial glaze layer; a common electrode (12) formed on the substrate and electrically connected to the heating resistor; and a plurality of individual electrode (13) formed on the substrate and electrically connected to the heating resistor. The common electrode includes a plurality of comb-like teeth (12A). Each of the comb-like teeth includes a tip portion (12c) having a smaller width and a base portion (12d) having a larger width. Each of the individual electrodes includes a tip portion (13d) having a smaller width and an intermediate portion (13e) having a larger width.

This application is the national stage application of internationalapplication number PCT/JP99/02131, filed on Apr. 22, 1999.

TECHNICAL FIELD

The present invention relates to a thick-film thermal printhead.

BACKGROUND ART

An example of a conventional thick-film thermal printhead is shown inFIG. 5 and FIG. 6. Each of these conventional thermal printheads(indicated by reference code P) comprises a rectangular head substrate1′ and a print substrate 2′, As shown in FIG. 5, the head substrate 1′has a first longitudinal edge 1 a′ and a second longitudinal edge 1 b′extending in parallel to each other. Further, the head substrate 1′ hasa first end 1 c′ and a second end 1 d′ extending between the first andthe second longitudinal edges. Likewise, the print substrate 2′ has twolongitudinal edges and two ends.

The head substrate 1′ has an upper surface entirely covered by a glazelayer 10′ (FIG. 6) made of amorphous glass. On an upper surface of theglaze layer 10′, a linear heating resistor 11′ extending along the firstlongitudinal edge 1 a′ is formed. The head substrate 1′ is furtherformed with a common electrode 12′ and a plurality of individualelectrodes 13′. As shown in FIG. 5, the common electrode 12′ extendsalong the first end 1 c′, the first edge 1 a′, and the second end 1 d′.Further, the common electrode 12′ has a plurality of comb-like teeth12A′ extending in parallel to each other. Each of the comb-like teeth12A′ has a tip potion 12 a′ contacting the heating resistor 11′.

Each of the individual electrodes 13′ has a first end portion 13 a′ anda second end portion 13 b′ away therefrom. The first end portion 13 a′contacts the heating resistor 11′ and extends between two adjacentcomb-like teeth 12A′ On the other hand, the second end portion 13 b′ isformed with a bonding pad 13 c′. The bonding pad 13 c′ is electricallyconnected to a drive IC 14′ via a connecting wire W′.

With the above constitution, the heating resistor 11′ is divided into aplurality of regions 15′ by the comb-like teeth 12A′. (FIG. 5 shows onlyone region 15′.) In each of the regions 15′, electric current is passedselectively via the drive IC 14′, to heat the selected region 15′,making each of the regions 15′ function as a heating dot.

The prior-art thick-film thermal printhead P as described above has afollowing disadvantage: Specifically, the thermal printhead P canprovide a good printing result if the printing is performed at a speedof about 2 inches per second (2 ips). However, if the printing speed isincreased to about 6 ips for example, printed image can be partiallyblurred, or an unintended whisker-like projection (feathering) can beprinted on a printing sheet.

DISCLOSURE OF THE INVENTION

A thick-film thermal printhead provided by a first aspect of the presentinvention comprises: an oblong rectangular substrate having at least onelongitudinal edge; a partial glaze layer provided on the substrate alongthe longitudinal edge; a linear heating resistor formed on the partialglaze layer; a common electrode formed on the substrate and electricallyconnected to the heating resistor; and a plurality of individualelectrodes formed on the substrate and electrically connected to theheating resistor.

According to a preferred embodiment, the partial glaze layer has anarcuate cross section. Further, the partial glaze layer has a thicknessof 10-25 μm and a width of 400-1000 μm.

Preferably, the common electrode includes a plurality of comb-like teetheach including a tip portion having a smaller width and a base portionhaving a larger width.

The tip portion of each comb-like tooth may be entirely formed on thepartial glaze layer. In this case, preferably, the base portion of eachcomb-like tooth is formed only partially on the partial glaze layer.

Preferably, the base portion of each comb-like tooth is spaced from theheating resistor.

Preferably, the base portion of each comb-like tooth extends on both ofthe partial glaze layer and the substrate.

According to the preferred embodiment, each of the individual electrodesincludes a tip portion having a smaller width for contact with theheating resistor, and an intermediate portion having a larger width.

Preferably, the intermediate portion of each individual electrode isspaced from the heating resistor.

Preferably, the intermediate portion of each individual electrodeextends on both of the partial glaze layer and the substrate.

Other object, characteristics and advantages of the present inventionwill become clearer from an embodiment to be described with reference tothe attached drawings.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

FIG. 1 is a plan view of a thick-film thermal printhead according to thepresent invention;

FIG. 2 is a plan view of a primary portion of the thick-film thermalprinthead in FIG. 1;

FIG. 3 is a sectional view taken in lines III—III in FIG. 2;

FIG. 4 is a graph showing a thermal response characteristic of a heatingdot;

FIG. 5 is a plan view of a prior art thick-film thermal printhead; and

FIG. 6 is a sectional view taken in lines VI—VI in FIG. 5.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a preferred embodiment of the present invention will bedescribed with reference to FIG. 1-FIG. 4.

FIG. 1 is a plan view showing a thick-film thermal printhead X accordingto the present invention. As shown in the figure, the thick-film thermalprinthead x comprises an oblong rectangular head substrate 1 and anoblong print substrate 2 mounted in adjacency thereto. The headsubstrate 1 is made of an electrically insulating material such asalumina ceramic whereas the print substrate 2 is made of an electricallyinsulating material such as glass epoxy resin.

As shown in FIG. 1, the head substrate 1 has a first longitudinal edge 1a and a second longitudinal edge 1 b extending in parallel to eachother. Further, the head substrate 1 has a first end 1 c and a secondend 1 d extending between the first and the second longitudinal edges.Likewise, the print substrate 2 has two longitudinal edges and two ends.

The head substrate 1 has an upper surface formed with a partial, linearglaze layer 10 made of amorphous glass. The partial glaze layer 10extends in parallel to the first longitudinal edge 1 a (and the secondlongitudinal edge 1 b), closer to the first longitudinal edge 1 a thanto the second longitudinal edge 1 b. The partial glaze layer 10 has athickness D1 (FIG. 3) of 10-25 μm, and a with D2 of 400-1000 μm.Advantages achieved from such an arrangement as this will be describedlater.

The partial glaze layer 10 can be formed by applying an amorphous glasspaste on the head substrate 1 and then baking the same. As shown in FIG.3, the partial graze layer 10 has a smooth arcuate upper surface. Thisis because the applied glass paste flows at the time of baking. Along apeak portion of the partial glaze layer 10, a linear heating resistor 11is formed.

The head substrate 1 is further formed with a common electrode 12 and aplurality of individual electrodes 13. As is clear from FIG. 1, thecommon electrode 12 extends along the first end 1 c, the first edge 1 a,and the second end 1 d. Further, the common electrode 12 has a pluralityof comb-like teeth 12A extending in parallel to each other. Each of thecomb-like teeth 12A contacts the heating resistor 11.

Each of the individual electrodes 13 has a first end portion 13 a and asecond end portion 13 b away therefrom. The first end portion contactsthe heating resistor 11 and extends between two adjacent comb-like teeth12A. On the other hand, the second end portion is formed with a bondingpad 13 c. The bonding pad 13 c is electrically connected to a drive IC14 via a connecting wire W.

As shown in FIG. 2, each of the comb-like teeth 12A includes a tipportion 12 c having a smaller width, and a base portion 12 d having alarger width. The tip portion 12 c is entirely formed on the partialglaze layer 10, and electrically contacts to the heating resistor 11. Onthe other hand, the base portion 12 d is spaced from the heatingresistor 11, and only a part of the base portion is formed on thepartial glaze layer 10. The other portion of the base portion 12 d isformed on the head substrate 1. The width of the tip portion 12 c is20-25 μm for example, whereas the width of the base portion 12 d is 80μm for example. The tip portion 12 c has a length of 400 μm for example.

Likewise, the first end portion of each of the individual electrodes 13includes a tip portion 13 d having a smaller width, and an intermediateportion 13 e having a larger width. The tip portion 13 d is entirelyformed on the partial glaze layer 10, and electrically contacts to theheating resistor 11. On the other hand, the intermediate portion 13 e isspaced from the heating resistor 11, and only a part of the intermediateportion is formed on the partial glaze layer 10. The other portion ofthe intermediate portion 13 e is formed on the head substrate. The widthof the tip portion 13 d is 20-25 μm for example, whereas the width ofthe intermediate portion 13 e is 80 μm for example. The tip portion 13 dhas a length of 400 μm for example.

With the above structure, the heating resistor 11 is divided into aplurality of regions 15 by the comb-like teeth 12A. (FIG. 2 shows onlyone region 15.) In each of the regions 15, electric current is passedselectively via the drive IC 14, to heat the selected region 15, makingeach of the regions 15 function as a heating dot. The number of theheating dots is varied in accordance with conditions such as the size ofrecording paper to be used. For example, if printing is to be made to anA-4 size recording paper at a printing density of 200 dpi, 1728 heatingdots are formed in a direction of secondary scanning.

The common electrode 12 and each of the individual electrodes 13 can beformed by using the following method: Specifically, first, a pastecontaining an electrically conductive metal such as gold is prepared.Next, the paste is applied on the head substrate 1, and then baked.Then, finally, the baked material is etched by means of photolithographyinto a predetermined pattern. According to such a method as above, thecommon electrode 12 and the individual electrodes 13 can be formedsimultaneously. The common electrode 12 and the individual electrodes 13have a thickness of about 0.6 μm.

The heating element 11 can be formed by first applying a resistor patecontaining ruthenium oxide on the partial glaze layer 10, and thenbaking the applied paste. The heating resistor 11 has a thickness ofabout 9 μm for example.

As shown in FIG. 3, a protective coating 16 is formed to cover theheating resistor 11, the common electrode 12 and each of the individualelectrodes 13. However, the bonding pads 13 c of the individualelectrodes 13 are not covered by the protective coating 16. Theprotective coating 16 can be formed by applying a glass paste on thehead substrate 1 and then baking the glass paste. The protective coating16 has a thickness of 4-8 μm for example.

Alternatively, the protective coating 16 can be formed by anelectrically conductive material such as Ti-sialon and SiC to athickness of 4-8 μm. In this case, the formation of the protectivecoating 16 is performed by using such a technique as sputtering andchemical vapor deposition (CVD) method.

As has been described earlier, in the thick-film thermal printheadaccording to the present invention, the heating resistor 11 is formed onthe partial glaze layer 10. Therefore, it becomes possible to make theheating resistor 11 appropriately contact the recording paper.

The thickness D1 of the partial glaze layer 10 is 10-25 μm, whereas thewidth D2 is 400-1000 μm. By making the partial glaze layer 10 into theabove given dimensions, thermal responsiveness of the heating resistor11 can be improved over that of the prior art. This point will bedescribed specifically hereafter.

Generally, the thermal responsiveness of the heating resistor 11decreases to deteriorate printing quality when the area of cross sectionof the partial glaze layer 10 increases. On the contrary, if the area ofcross section of the partial glaze layer 10 is too small, the heatingresistor 11 does not properly contact the recording paper. The inventorof the present invention has found that these problems can be eliminatedby setting the thickness and the width of the partial glaze layer 10 tothe values given above. The inventors of the present invention conductedexperiments, with results shown in the table below. (The experimentswere made with thermal printhead each having a printing density of 200dpi, and printing was performed at a speed of 6 ips. The commonelectrode and the individual electrodes of each thermal printhead wereformed by using gold to a thickness of 0.6 μm. The heating resistor wasmade from a resistor paste containing ruthenium oxide to a thickness of9 μm.)

Thermal Thick- Response Glaze ness Width Time Printing Type [μm] [μm](t:msec) Quality Example Partial 12 400 0.63 Good 1 Glaze No blur Nofeathering Example Partial 24 800 0.85 Good 2 Glaze No blur Nofeathering Example Partial 50 800 1.20 No good 3 Glaze Some blur &Feathering Example Entire 10 — 0.56 No Good 4 Glaze some blur &Feathering

As understood from the Table, the thermal responsiveness of the heatingresistor increases if the thickness of the partial glaze layer is 10-25μm and the width thereof is 400-1000 μm, and as a result, good printingimage is obtained. It should be noted here that, as shown in FIG. 4, thethermal responsiveness of the heating resistor is evaluated on the basisof time T which is the time necessary for a surface temperature of theheating resistor to descent from 300° C. to 100° C. Specifically, theshorter is the time T, better is the thermal responsiveness.

The thick-film thermal printhead according to the present inventionfurther has the following advantages: Specifically, as has beendescribed with reference to FIG. 2, each of the comb-like teeth 12A andthe individual electrodes 13 contacts the heating resistor 11 via thecorresponding tip portion 12 c or 13 d which has the smaller width.According to such an arrangement as this, the area of each heating dot15 can be increased than in the prior art, without decreasing thedensity of the heating dots 15.

Further, according to the present invention, rupture of each comb-liketooth 12A (or the individual electrode 13) can be effectivelyeliminated. Specifically, there is a step between the head substrate 1and the partial glaze layer 10, and therefore the comb-like tooth 12A isformed as folded on the head substrate 1 and the partial glaze layer 10(FIG. 3). Because stress concentrates onto such a folded portion asabove, the folded portion is relatively easily ruptured. However,according to the present invention, the folded portion is the wider baseportion 12 d. Therefore, even with the stress concentration, thecomb-like tooth 12A is not ruptured easily, and this also applies toeach of the individual electrodes.

What is claimed is:
 1. A thick-film thermal printhead comprising: anoblong rectangular substrate (1) having at least one longitudinal edge(1 a); a partial glaze layer provided on the substrate along thelongitudinal edge; a linear heating resistor (11) formed on the partialglaze layer; a common electrode (12) formed on the substrate andelectrically connected to the heating resistor; and a plurality ofindividual electrodes (13) formed on the substrate and electricallyconnected to the heating resistor; wherein the common electrode has aplurality of comb-like teeth (12A) contacting the heating resistor, eachof the comb-like teeth including a tip portion (12 c) having a smallerwidth and a base portion (12 d) having a larger width, and wherein thelarger-width base portion of each comb-like tooth extends on both of thepartial glaze layer and the substrate while crossing a longitudinal edgeof the partial glaze layer.
 2. The thick-film thermal printheadaccording to claim 1, wherein the partial glaze layer has an arcuatecross section.
 3. The thick-film thermal printhead according to claim 1,wherein the partial glaze layer has a thickness of 10-25 μm and a widthof 400-1000 μm.
 4. The thick-film thermal printhead according to claim1, wherein the base portion of each comb-like tooth is spaced from theheating resistor.
 5. A thick-film thermal printhead comprising: anoblong rectangular substrate (1) having at least one longitudinal edge(1 a); a partial glaze layer provided on the substrate along thelongitudinal edge; a linear heating resistor (11) formed on the partialglaze layer; a common electrode (12)formed on the substrate andelectrically connected to the heating resistor; and a plurality ofindividual electrodes (13) formed on the substrate and electricallyconnected to the heating resistor; wherein the common electrode has aplurality of comb-like teeth (12A) contacting the heating resistor, eachof the comb-like teeth including a tip portion (12 c) having a smallerwidth and a base portion (12 d) having a larger width, and wherein thesmaller-width tip portion of each comb-like tooth is entirely formed onthe partial glaze layer and does not cross a longitudinal edge of thepartial glaze layer.
 6. A thick-film thermal printhead comprising: anoblong rectangular substrate (1) having at least one longitudinal edge(1 a); a partial glaze layer provided on the substrate along thelongitudinal edge; a linear heating resistor (11) formed on the partialglaze layer; a common electrode (12)formed on the substrate andelectrically connected to the heating resistor; and a plurality ofindividual electrodes (13) formed on the substrate and electricallyconnected to the heating resistor; wherein the common electrode has aplurality of comb-like teeth (12A) contacting the heating resistor, eachof the comb-like teeth including a tip portion (12 c) having a smallerwidth and a base portion (12 d) having a larger width, and wherein thelarger-width base portion of each comb-like tooth is formed onlypartially on the partial glaze layer and crosses a longitudinal edge ofthe partial glaze layer.
 7. A thick-film thermal printhead comprising:an oblong rectangular substrate (1) having at least one longitudinaledge (1 a); a partial glaze layer provided on the substrate along thelongitudinal edge; a linear heating resistor (11) formed on the partialglaze layer; a common electrode (12)formed on the substrate andelectrically connected to the heating resistor; and a plurality ofindividual electrodes (13) formed on the substrate and electricallyconnected to the heating resistor; wherein each of the individualelectrodes includes a tip portion (13 d) having a smaller width and anintermediate portion (13 e) having a larger width, and wherein theintermediate portion of each individual electrode extends on both of thepartial glaze layer and the substrate while crossing a longitudinal edgeof the partial glaze layer.
 8. The thick-film thermal printheadaccording to claim 7, wherein the partial glaze layer has an arcuatecross section.
 9. The thick-film thermal printhead according to claim 7,wherein the partial glaze layer has a thickness of 10-25 μm and a widthof 400-1000 μm.
 10. The thick-film thermal printhead according to claim7, wherein the intermediate portion of each individual electrodes isspaced from the heating resistor.